ISM Policy Update Not Reflected on Older Managed Indices After Replica Count Change

Plugins Index Management
1

Versions (relevant - OpenSearch/Dashboard/Server OS/Browser): 3.6.0

Describe the issue:

I have an Index State Management (ISM) policy that manages index lifecycle across Hot, Snapshot, Warm, and Delete states.

Originally, the policy was configured to set number_of_replicas: 2 when an index transitioned to the Warm state. After the policy had already been managing indices for some time, Then i updated the policy and changed the Warm state replica to 0

"replica_count": {
  "number_of_replicas": 0
}

The policy update was successfully saved and the current policy definition reflects the new value.

However, I observed that some older managed indices continued to behave as if they were using the previous policy version, while newer indices appeared to use the updated policy.

Current policy metadata:

"_seq_no": 239501,
"_primary_term": 16

Warm state action:

{
  "name": "warm",
  "actions": [
    {
      "allocation": {
        "require": {
          "temp": "warm"
        }
      }
    },
    {
      "replica_count": {
        "number_of_replicas": 0
      }
    }
  ]
}

What I Observed

For an older managed index:

GET _plugins/_ism/explain/prod-trx-logs-2026.05.22

The response shows:

{
  "policy_id": "hot-warm-policy",
  "policy_seq_no": 91125,
  "policy_primary_term": 1,
  "state": {
    "name": "warm"
  }
}

The policy sequence number associated with the index is significantly older than the current policy sequence number.

For newer indices managed by the same policy, the reported:

"policy_seq_no"

matches the currentt policy version:

239501

Is this an expected behaviour ?

thanks,

Jijo

2

Does it mean that you have older indices with the latest policy ID?

3

No , older indices are with old policy version and new indices are showing new version. so when updating the policy , it is not getting updated to existing index

4

Updates to policies aren’t automatically applied to existing indices.
Through Dashboards you can go into Index Management → Policy managed indexes and chick Change policy at the top right. There you can choose indices, or some_pattern* and change to a “new” policy with the same name.

5

Thanks @reshippie , let me check that option